Various processes are known in the art that involve the production of metal oxides from vaporous reactants. The most basic requirements of such processes necessitate a feedstock solution, a means of generating and transporting vapors of the feedstock solution (hereafter called vaporous reactants) and an oxidant to a reaction site, and a means of catalyzing oxidation and combustion coincidentally, producing finely divided, spherical aggregates, called soot. This soot may be collected in any number of ways, ranging from a collection chamber to a rotating mandrel, and simultaneously or subsequently heat treated to form a non-porous, transparent, high purity glass article. The means for executing these reactions is usually a specialized piece of equipment with a unique arrangement of nozzles and burners.
Much of the initial research that led to the development, and thus patent protection, of a plethora of such processes focused on the production of fused silica. Selection of the appropriate feedstock was found to be as important in the production of high purity fused silica as the equipment used in its production. Consequently, a material was identified that could generate the needed vapor pressure of 200-300 mm at temperatures below 100° C.; the high vapor pressure of silicon tetrachloride (SiCl4) isolated it as a convenient vapor source for soot generation, thus launching the discovery and use of a series of similar chloride-based feedstocks. This factor, more than any other, is responsible for the presently accepted use of SiCl4, GeCl4, POCl3, and BCl3 as vapor sources, even though these materials have certain chemically undesirable properties.
Silicon, germanium, zirconium, and titanium are metals often used in halide form as vaporous reactants for forming high purity metal oxide glasses. However, SiCl4 has been the industry standard among metal-source vaporous reactants used over the years for the production of high purity silica glasses. As disclosed in U.S. Pat. No. 3,698,936, one of several reactions may be employed to produce high purity fused silica via oxidation of SiCl4; namely:    (1) SiCl4+O2→SiO2+Cl2,    (2) SiCl4+O3→SiO2+Cl2, or    (3) SiCl4+H2O→SiO2+HCl,whereby burners or jet assemblies are utilized in feeding the reactant gases and vapors to a reaction space. There are inherent economic disadvantages to each of these reactions.
These reactions, which oxidize SiCl4 through pyrolysis and hydrolysis, have the disadvantage of producing a very strong acid by-product. While the first two reactions occur theoretically, it is likely that an auxiliary fuel is needed to achieve pyrolytic temperature, thus leading to hydrolysis of the silicon tetrachloride and formation of hydrochloric acid (HCl). Such a by-product is not only a detriment to many deposition substrates and the reaction equipment, but also is a detriment to the environment. Emission abatement systems have proven to be very expensive due to down-time, loss, and maintenance of equipment caused by the corrosiveness of HCl.
The first reaction, which utilizes oxygen as it occurs naturally, requires elevated reaction temperatures which, generally, are difficult to maintain without using specialized equipment. The second reaction requires ozone, an unstable form of molecular oxygen that not only warrants special handling, but also must be manufactured on site due to a lack of commercial availability. Notwithstanding the handling and disposal of the HCl by-product necessitated by the hydrolysis and pyrolysis of SiCl4, the third reaction, also hydrolysis of SiCl4, tends to be the preferred commercial method of producing fused silica for economic reasons.
Though hydrolysis of SiCl4 has been the preference of industry for producing high purity fused silica over the years, the enhanced global sensitivity to environmental protection has led to more strict government regulation of point source emissions, prompting a search for less environmentally pernicious feedstocks. In new point source emission regulations, HCl, the by-product of hydrolyzing SiCl4, as well as many particulate pollutants, has to be cleansed from exhaust gases prior to their release into the atmosphere. The economic consequences of meeting these regulations have made commercial production of fused silica by downstream removal of HCl and other metal oxides from halide-based feedstocks less attractive to industry.
As an alternative, high purity fused quartz or silica may also be produced by thermal decomposition and oxidation of silane, a compound that requires taking safety measures in handling due to the violent reaction caused when air is introduced into a closed container of silane. Silane is commonly reacted with carbon dioxide, nitrous oxide, oxygen, or water to produce a high purity material that is useful in producing, among other things, semiconductor devices. However, silane has proven to be much too expensive and reactive to be considered for commercial use except possibly for extremely high purity applications.
The novelty of the invention described herein lies in the replacement of SiCl4 in vapor deposition processes with a halide-free, silica-source compound, thus greatly reducing, if not eliminating, the production of HCl. The advantages of operating under a halide-free system include: reduced pollution abatement requirements and reduced equipment losses and maintenance due to the corrosive nature of HCl.
The teachings of the instant invention are easily adapted to known methods of producing high purity fused silica by flame pyrolysis or hydrolysis, such as those disclosed in the early patents by Nordberg (U.S. Pat. No. 2,239,551) in 1941 and Hyde (U.S. Pat. No. 2,272,342) in 1942. It is anticipated that this process alteration may be adapted to a variety of deposition/collection techniques as well. Therefore, it is an object of this invention to provide an improved method of making high purity fused silica by utilizing alternative silicon-source compounds, thus greatly reducing, if not eliminating, the need for elaborate pollution abatement equipment.
While it is recognized that the primary application of the instant invention relates to the production of fused silica, the technology applied herein is generally applicable in instances where a high purity metal oxide glass is desired.
It is a further object of this invention to provide an improved method of making high purity metal oxide glasses through the use of alternative metal oxide source compounds, thereby greatly reducing the need for expensive pollution abatement systems.